1. Field of the Invention
The present invention is directed to a distribution system for supplying backwash gas and liquid to a gravity filter of the type having a granular filter bed through which a liquid to be treated may flow downwardly.
2. Description of the Prior Art
Granular media filters, which are commonly used for the purification of water, typically comprise a carefully graded filter bed of particulate media such as sand, anthracite, or other known materials. The filter bed is supported above a floor by an underdrain which serves as an outlet for filtered effluent exiting the bed and an inlet for backwash fluids. In use, as the water to be treated is passed downwardly through the filter bed, the suspended solids are removed from the liquid and retained within the granular media. Periodically, it is necessary to remove the suspended solids collected in the filter bed by backwashing the filter bed. Backwashing is conventionally achieved by passing backwash fluids upwardly through the filter media, loosening and removing the suspended solids to carry them to the top of the filter bed. It is desirable that the backwash fluids, water and optionally air, be distributed evenly throughout the filter bed. Uneven distribution of the backwash fluids may cause shifting in the graded filter bed media, thereby decreasing the effectiveness of the filter bed and possibly leaving portions of the filter bed uncleaned.
Various cleaning systems for gravity filters have been devised in the prior art. For example, U.S. Pat. No. 2,710,692 discloses a filter cleaning system including a gas supply pipe 5 extending the length of the flume. Distribution holes in the gas distribution pipe 5 distribute backwash gas from the pipe into the filter bed. The difficulty with this apparatus is that the distribution pipe takes up water flow area in the flume, thereby limiting the operational rate of the filter during normal filtering operations.
U.S. Pat. No. 3,512,649 discloses another system for supplying backwash gas and liquid to a gravity filter bed. The backwash gas is delivered through a series of independent pipes running throughout the filter media support. The difficulties with this system include the extensive labor required in installing such a system, the added cost of materials for such a system, and the increased likelihood of corrosion or fouling due to the multiplicity of pipes.
Another system for cleaning gravity filters is disclosed in U.S. Pat. No. 3,831,761. This system supplies backwash gas through a central distributor pipe in which each conduit 10 has an associated dip tube, or downpipe, providing communication between the conduit and the backwash gas distributor pipe 15. The gas distributor pipe in this system is also located in the flume or exhaust chamber which, as discussed above, may place operational limitations on the filter. Furthermore, the individual downpipes are expensive as well as difficult to install. A similar system is disclosed in U.S. Pat. No. 3,468,422, wherein backwash gas is supplied to each individual lateral from a main air header pipe 56 through a series of J-shaped piping branches, at least one of which extends to each individual lateral. Here again, the air header piping takes up needed water flow area in the flume, the air header piping may be expensive, and such piping and multiple individual branches are difficult to install.
Finally, the air/water underdrain backwash system disclosed in U.S. Pat. No. 4,331,542 provides separate liquid and gas systems for backwashing. The duplication of a second system to supply backwash gas can be expensive, and since the gas and the liquid do not mix until they are both in the filter bed itself, a thorough, even cleaning is not possible.
It is therefore an object of this invention to maximize water flow area in the flume by the elimination of the air header pipe previously positioned in the flume. It is a further object of this invention to minimize the cost of the underdrain system by minimizing the amount of piping needed and simplifying the installation of the underdrain. It is a still further object of this invention to minimize or eliminate filter dead spaces where no backwash air/water flows.